The present invention relates to a drive train for a motor vehicle which has a front axle and a rear axle, of which one is driven constantly and the other is driven as required, with a drive unit which is installed in the motor vehicle transversely at the front and provides drive torque via an output member, the output member being connected to the constantly driven axle, with an angular gear which is arranged in the region of the front axle and is connected to a cardan shaft which serves for transferring drive torque to the rear axle, and with a friction clutch arrangement for cutting in the axle driven as required.
The present invention relates, furthermore, to a power divider for a drive train, with a casing, from which a first and a second drive shaft emerge in the lateral direction and from which a cardan shaft emerges in the longitudinal direction, with an angular gear arranged in the casing and with a friction clutch arrangement which can be actuated by means of a clutch actuator.
In motor vehicles, the four-wheel drive was originally used virtually exclusively in all-terrain vehicles. In recent years, however, even vehicles predominantly designed for road traffic (such as, for example, passenger cars in the form of limousines, station wagons, SUVs) have often been equipped with four-wheel drive, specifically to increase driving safety, particularly also in the case of unfavorable traction conditions.
In vehicles with four-wheel drive, a distinction is generally made between differential-controlled systems and clutch-controlled systems. In differential-controlled systems, the drive torque is apportioned to the front axle and the rear axle from the drive unit by means of a differential. In clutch-controlled systems, generally only one axle is driven and the other axle is driven as required. In this case, in the simplest instance, a manual-shift clutch is provided which is actuated from the passenger interior. However, modern vehicles of this type with four-wheel drive use clutches actuated in an automated way (for example, Haldex clutches) which cut in the second axle when a differential rotational speed builds up or has built up between the axles. These systems are also designated as hang-on systems.
Where differential-controlled systems are concerned, it is known, in the event of the absence of traction, to block the longitudinal differential via a clutch so that torque is transferred to the axle having the higher coefficient of friction.
Where clutch-controlled systems are concerned, there are known systems with hang-on in relation to the rear axle. These are generally vehicles with front-wheel drive, the rear axle being allocated torque in the event of an absence of traction at the front axle.
Such a drive train is installed, for example, in the Audi TT. In this case, in the rear region of the cardan shaft, upstream of the rear-axle differential, a friction clutch arrangement in the form of a Haldex clutch is provided, by means of which the rear axle is cut in as required.
Conversely, systems are also known in which, in general, the rear axle is driven and the front axle is designed as a hang-on axle.
A drive unit is understood in the present context to mean a unit for the provision of drive torque. This may be an engine, such as, for example, an internal combustion engine, or an electric motor, either as such or in combination with a transmission.
The transmission may be a manual-shift multi-step transmission, an automatic converter assembly, a double-clutch transmission, an automated shift transmission, a continuously variable transmission, etc.
A series of concepts is known with regard to power dividers for motor vehicles with four-wheel drive.
For example, from DE 37 21 628 C2 it is known to arrange coaxially with respect to the front axle a transverse differential and a longitudinal differential which are both designed as bevel wheel differentials. In this case, an output of a drive unit transversely installed at the front is connected via a hollow shaft structure to an input member of the longitudinal differential. The latter, in turn, is connected to the outer circumference to a crown wheel of an angular gear which drives a cardan shaft for the rear axle.
Furthermore, it is known from WO 02/28678 A1 to provide coaxially with respect to the front axle a longitudinal and a transverse differential which are designed in each case as planet wheel differentials. The planet wheel differentials are in this case received in a transmission casing. A casing for receiving an angular gear for the output to the rear axle is flanged laterally to the transmission casing.
DE 103 13 386 A1 discloses a further power divider for a drive unit transversely installed at the front. In this case, a longitudinal differential designed as an epicyclic gear is provided, the output member of which is in engagement with a gearwheel of a hollow-shaft stub which is arranged parallel to the front axle. A bevel wheel is secured to another end of the hollow-shaft stub and drives a cardan shaft for the rear axle.
Hang-on systems present the problem of vibrations which are triggered by the friction clutch arrangement for cutting in the axle driven as required. Moreover, as a rule, the power dividers are special structures which are suitable solely for a specific drive train.